If left on beaches or in parks, the cups break down into small pieces that Environment Texas director Luke Metzger said may be mistaken by wildlife for food. Some research even says Texas has the most polluted beaches in the U.S.

"Polystyrene is one of the worst forms of plastic pollution," Metzger said. "Polystyrene is continuing to be a major source of litter and is an eyesore in our parks and rivers and of course our beaches on the Gulf."

Last month, the group sent a letter to Whataburger's main offices in San Antonio pressuring them to stop using polystyrene.

Metzger said they never heard back.

To follow-up, the group began protesting outside the company's corporate office in San Antonio Tuesday and continued Wednesday.

If Whataburger decides to discontinue the cups, the company will be following in McDonalds' and Dunkin' Donuts' footsteps. Both recently committed to phasing out polystyrene cups within the next few years.

"Many of us love the company but have been disappointed by their continued use of polystyrene especially in the wake other restaurant chains committing to phase out the use of polystyrene," Metzger said.

In a statement, Whataburger said it is looking at cup alternatives that would still keep drinks at the right temperature.

"We know our customers have come to expect high quality standards from us and will share any updates when we have news to share," Whataburger Corporate Communications said. "Our Styrofoam cups are recyclable and we encourage everyone to dispose of them properly."